The present invention relates to a color image processing method and apparatus for converting three color signals received from a color image input device or a color image generation device to color image recording signals of four colors including black. More specifically, the invention relates to a color image processing method and apparatus for producing a desired print output from various types of color signals.
In conventional color printers, color copiers, etc., color conversion processing is performed so as to conform to the color reproduction characteristics of a subject recording device by restricting input colors. For example, in a color copier which incorporates an input device, a print output is produced from a particular input document and color conversion processing from input color signals to recording color signals is performed so that the print output color coincides with the document color. On the other hand, in a color printer, a color conversion process from input color signals to recording color signals is set with an assumption (restriction) that certain representative signals are input. In general, the representative input signals are NTSC RGB television signals. The color coincidence between a display and a print output is attained by properly setting a conversion process from the NTSC RGB signals to recording signals of cyan, magenta, yellow and black.
However, now, a variety of color input and output media are used and color signals are exchanged between those media through a network, and color signals are diversified accordingly. For example, this situation is reflected in the types of application software for editing color image information. FIG. 2 shows specifications of color expression in typical pieces of application software. All of the following color spaces can be used: (1) RGB space, (2) HSL and HSB spaces defined by operations of modifying the RGB space, and (3) CMYK space using recording colors themselves. In general, where the final outputs are printing outputs, CMYK signals are used which are sent to a plate making scanner for printing.
It is a commonly known case that, even if the same color space is used, actual color data are different. For example, the RGB signals of general color scanners are different from the NTSC RGB signals. Further, the RGB signals are different among a plurality of color scanners because of differences of spectral responses, for instance. Similarly, on the CMYK space, even if the same CMYK signals are used, different sets of colorants produce different print colors. That is, other than the color space, there exists an indicator of whether the color signals depend on devices. The "device-independent" signals mean signals which are convertible to signals on a colorimetric color coordinate space (e.g., CIE XYZ, L*a*b* and L*u*v*) according to known definitive equations. The NTSC RGB signals are device-independent signals. Conversely, the "device-dependent" signals are signals which are set assuming characteristics of a particular device. The CMYK signals and the RGB signals of color scanners are device-dependent signals. To process device-dependent signals with another type of device, it is necessary to describe some corresponding relationship between the device-dependent signals and device-independent signals. Examples of those relationships are shown in FIG. 2 in the column of color matching, which gives color coordinates for plural points of CMYK signals. As shown in FIG. 2, there exist a plurality of color matches in association with the kinds of printing inks, etc.
Even now, there has not been proposed a color conversion process which can process a number of kinds of input color signals independently of the kinds to provide faithful reproduction. One method of realizing such a process by use of approximation is such that in a known matrix-type color conversion process, plural sets of conversion coefficients are prepared for different kinds of input color signals and switched in accordance with the kind of input color signals. However, this method cannot provide sufficient color reproduction accuracy because of nonlinearity of recording devices, and cannot extend the conversion in a simple manner for input color signals of four colors such as CMYK signals.
A direct look-up table type color conversion method is known as a method of providing more faithful reproduction. However, to apply this method to a number of kinds of input color signals, conversion tables of the number of kinds of input color signals need to be stored, which increases the memory cost. Further, like the above matrix type conversion method, in the case of input color signals of four colors such as CMYK signals, the memory size and the size of interpolation operation become so large as to disable simple extension of the conversion.